Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

Abstract

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB 991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0.^primeprime6 away. The spectrum shows a depression in flux in between 4000 Å and 5500 Å. This could be the result of a 2175 Å-type extinction feature in the host of GRB 991216, but at a rather red wavelength of 2360 Å. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB 011211 are identified with lines originating in a single absorption system at z=2.142±0.002, the redshift of the GRB 011211 host galaxy. We also detect the Lyα absorption line in the host, to which we fit a neutral hydrogen column density of log N(ion{H}{i}) = 20.4±0.2, which indicates that it is a damped Lyα system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB 011211 redshift to be [Si/H] = -0.9^+0.6_-0.4, [Fe/H]=-1.3±0.3, and [Al/H] = -1.0^+0.5_-0.3. For GRB 021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [ion{O}{ii}] λ3727 at z=1.006. The corresponding unobscured [ion{O}{ii}] star-formation rate is 1.4 M⊙ yr^-1.